1. Field of the Invention
The present invention generally relates to a laminated sheet, a method of producing the laminated sheet, an exhaust gas processing device including the laminated sheet as a holding sealer, and a method of producing the exhaust gas processing device.
2. Description of the Related Art
The number of cars in the world has greatly increased since the beginning of this century, and along with the increase in the number of cars, the amount of exhaust gas from internal combustion engines of cars has greatly increased. Especially, exhaust gas from diesel engines contains various pollutants and is a serious threat to the world environment.
To reduce pollution resulting from exhaust gas, various exhaust gas processing devices have been proposed and put into practical use. Normally, an exhaust gas processing device is provided in the path of an exhaust pipe connected to an exhaust gas manifold of an engine, and includes a casing composed, for example, of metal and an exhaust gas processing unit disposed in the casing and having a large number of cells separated by cell walls. In many cases, these cells are arranged to form a honeycomb structure, and in such cases, the exhaust gas processing unit is simply called a “honeycomb structure”. Examples of exhaust gas processing units include catalyst carriers and exhaust gas filters such as a diesel particulate filter (DPF). Taking a DPF as an example, when exhaust gas passes through the cells of a honeycomb structure (or an exhaust gas processing unit), fine particles (particulates) are trapped by the cell walls and removed from the exhaust gas. Examples of materials for an exhaust gas processing unit include metals, alloys, and ceramics. Honeycomb filters composed of cordierite are popular as ceramic exhaust gas processing units. In these days, in terms of heat resistance, mechanical strength, and chemical stability, porous silicon carbide sintered bodies are widely used as materials for exhaust gas processing units.
Normally, a holding sealer is provided between an exhaust gas processing unit and a casing. A holding sealer prevents an exhaust gas processing unit from hitting the inner side of a casing when, for example, an automobile is running and thereby prevents damage to the exhaust gas processing unit. Also, a holding sealer prevents leakage of exhaust gas through a gap between an exhaust gas processing unit and a casing. Further, a holding sealer prevents an exhaust gas processing unit from falling off due to exhaust gas pressure. Meanwhile, to maintain reactivity of an exhaust gas processing unit, it is necessary to keep the exhaust gas processing unit at high temperature. For this purpose, a holding sealer must also have good thermal insulation performance. Holding sealers composed of inorganic fibers, such as alumina fibers, are known to satisfy requirements as described above.
An exemplary holding sealer composed of inorganic fibers is wound around at least a portion, other than the openings, of the outer surface of an exhaust gas processing unit, with two ends of the holding sealer being joined together. The holding sealer is fixed with tape to the exhaust gas processing unit. The exhaust gas processing unit wrapped by the holding sealer is inserted into a casing to produce an exhaust gas processing device.
Recent internal combustion engines emit exhaust gases at high temperatures and pressures. For this reason and to achieve the goals mentioned below, it is necessary to improve the thermal insulation performance of holding sealers.
(i) To prevent creation of a gap between a holding sealer and a casing which is caused by expansion of the casing due to heat transferred from the exhaust gas processing unit through the holding sealer.
(ii) To prevent degradation due to heat of parts (for example, instruments) attached to the outer surface of a casing.
(iii) To improve efficiency of a recycling process for certain types of exhaust gas processing units such as DPFs (in a recycling process, trapped particulates are combusted at high temperature to enable reuse of exhaust gas processing units).
One way to achieve the above goals is to increase the gap between an exhaust gas processing unit and a casing and increase the thickness of a holding sealer to improve its thermal insulation performance. However, as the thickness of a holding sealer increases, the difference between the outer circumference and the inner circumference of the holding sealer, when wound around an exhaust gas processing unit, increases. A larger difference between outer and inner circumferences increases the possibility of developing cracks in the outer surface (that is opposite to a surface in contact with the exhaust gas processing unit) of a holding sealer. Such cracks in turn lead to leaks of unprocessed exhaust gas.
Patent document 1 discloses an exhaust gas processing device including an exhaust gas processing unit and a holding sealer. Multiple grooves are formed on a surface (inner surface), which is in contact with the exhaust gas processing unit, of the holding sealer to prevent development of cracks resulting from the difference between outer and inner circumferences. The grooves formed on the holding sealer reduce the influence of the difference between outer and inner circumferences and thereby prevent development of cracks as well as resulting leaks of unprocessed exhaust gas.
[Patent document 1] Japanese Patent No. 3072281